Paper is principally manufactured from wood fibers. Two broad types of papermaking stock are used: mechanical pulp created by abrading raw wood to separate fibers, and chemical pulps which are produced by digesting wood chips in chemical liquor which dissolves the lignin which binds the wood fibers together.
The output from a chemical digester is a high consistency stock of fibers suspended in a solution containing dissolved lignin and digesting liquids often containing alkaline chemicals. Before further processes can be performed on the fibers separated from the wood chips the dissolved lignin and digesting chemicals, referred to as black liquor, must be separated from the fibers. To minimize downstream problems, and the production of undesirable waste products, the better than 99 percent of the black liquor must be separated from the fibers. The process of separating the dissolved lignin and digesting chemicals is referred to as the washing process.
The process is complicated by a need to minimize the dilution of the black liquor during the washing process. Dilution must be minimized because after separation of the fiber, the black liquor and all wash water are distilled and the residue is burnt to produce energy and ash. The ash, referred to as smelt, contains an alkaline residue which is processed to create the digesting liquor thus completing the digestion cycle by recycling the chemicals used to digest the lignin in the wood chips.
A paper manufacturing plant may produce 1,500 gallons per minute or more of black liquor wash water. Even with multistage distillation the energy demands for processing this quantity of liquid are high. To maximize washing effectiveness, while at the same time minimizing the amount of water used, counterflow washing is used.
Counterflow is an engineering technique wherein two process streams interact as they move in opposite directions. As applied to pulp washing this means a series of washers is set up with the final wash being performed with clean water. The waste water from the last washer is then used to wash the stock in the second from the last washer. Water from the second to last washer is used to wash stock in a third to last washer and so on for the total number of washers used.
The typical washer used by industry is a rotating cylinder which has a filter wire or cloth wrapped around the cylindrical surface. The rotating cylinder is submerged in a container of fiber stock typically having a consistency of about one percent. Vacuum is drawn on the inside surface of the cylinder drawing liquid through the filter and forming a fiber mat on the outside of the cylinder. Alternatively the rotating cylinder and fiber container are enclosed in a pressurized container and the interior of the cylinder is vented to draw stock through the filter.
The filter cloth or wire is supported on a corrugated deck. The cylindrical filter surface is divided into sections along the circumference of the cylinder, so that the vacuum drawn on a particular section can be turned off to allow the fiber mat to be removed from the filter surface. The division of the cylindrical surface into sections is accomplished by baffles which extend between sections. The drum has an inner cylindrical shell which is spaced from the corrugated deck forming a radially and axially extending space between the corrugated deck and the inner cylindrical shell. This space is divided by radial baffles into drainage sectors, one for each filter section.
Each sector drains down to the axis of the cylinder to a valve housing referred to as a grapefruit. At the grapefruit a stationary valve member controls which sectors are supplied with vacuum and which are not. Black liquor and/or wash water is drawn through a hollow shaft in the middle of the trunnion, or bearing supporting rotation of the drum. The hollow shaft connects to radially extending tubes which connect with each drainage sector. The drainage sectors in turn draw liquid through small openings called drainage louvers in the corrugated deck.
The liquid extracted from inside the drum is used to dilute the incoming pulp from the digester which has a consistency of 12-16 percent down to the one percent necessary to form a uniform fiber mat on the cylinder surface. A portion of the liquid extracted from the first washer is sent to the evaporators. Logically, the flow of black liquor and wash water sent to the evaporators is substantially equal to the volume of the wash water because the consistency of the pulp entering and leaving the washers is substantially the same. For a perfect washer therefore, the black liquor would not be diluted at all and would only be displaced by the wash water. Practical systems may result in the black liquor being diluted only twenty to thirty percent by the addition of wash water. Each subsequent washer utilizes the liquid extracted to dilute the incoming stock with the remaining stock flowing back to the previous washer. Typically three or four washers are required to adequately clean the black liquor from the digested pulp.
A number of approaches have been attempted to perform more than one washing step on a single washer.
One approach is to use a Fourdrinier washer where a continuous wire or forming fabric similar to that used to form a paper web is used. This approach is workable but requires typically five to seven wash stages because of the tendency for wash water flowing through the fiber mat to form paths of lower resistance, known as channeling, so that each step is less effective.
Reducing the total number of washer units from three or four units to two units would result in considerable cost savings. There is cost savings both in reduced capital and facility costs and maintenance and operating costs. What is needed is a cylindrical brown stock washer which can perform two washing steps on a single machine.